This invention relates generally to receivers, and more specifically to a multifrequency signal receiver for receiving and separately detecting several electrical signals of different frequencies transmitted through a single transmission medium or channel such as, typically, the usual telephone line.
Multifrequency signaling systems have been finding everincreasing use in recent years. U.S. Pat. No. 3,076,059, issued to L. A. Meacham et al., for example, discloses an example of such multifrequency signaling systems which can be utilized in telephone subscriber signaling systems. This prior art system includes a receiver capable of receiving multifrequency signals transmitted as from pushbutton telephone transmitters. For example, upon depression of the buttons marked 1, 2, 3 and so forth on the pushbutton telephone, electrical signals are generated which have frequencies corresponding to the respective buttons depressed, and these signals are transmitted to, and detected by, the receiver. The detected signals may be utilized by the receiver for the delivery of the corresponding outputs to a computer or to an automatic telephone switchboard. The system is also adaptable for end-to-end transmission of various information signals.
The transmission of multifrequency signals over telephone lines has the problem that the signal receiver tends, erroneously, to detect noise or voice signals traveling the same lines as the valid signals. According to one of the several proposals heretofore made to overcome this problem, two signals having appreciably different frequency components are employed to represent in combination each valid signal to be transmitted, and the receiver receives the two signals to detect the desired valid signal therefrom. The receiver is then less likely to make the noted erroneous signal detection that in the case where each incoming signal represents a different piece of information to be detected.
This proposal, however, also has its own disadvantages. If, during transmission of an information signal of a certain frequency, noise or voice signal is generated which has the same frequency component as the frequency of some other information signal, the receiver will detect such noise just as it does the valid information signal. Also, if two signals having different frequency components originate simultaneously due to noise or voice signals, then the receiver will erroneously derive some piece of information from the two signals. Outputs produced by the receiver as a result of inputs other than the valid information signals, of course, cause erroneous operation of the computor or the like connected in the subsequent stage of the receiver.